A spokesperson for El Paso Corp. confirmed that a third-party's bulldozer had hit the pipeline at about 2 p.m. Saturday, so the company "immediately" took steps to turn off the valves near the weakened part of the line.

"We immediately started to isolate the rupture, and then about 6:30, it ignited," said the spokesperson, who asked to remain anonymous. "As soon as we were notified there was an incident, we began our shut-down procedures."

She declined to answer further questions.

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Grass fires spread across a three-square mile rural area, away from any homes or businesses. Officials of the Texas Railroad Commission, which regulates oil and gas pipelines, were on the scene to investigate.

Lt. John Martin, a spokesman for the Harris County Sheriff's Office, said the explosion and fire were so intense that one captain reported that because of the heat he could not get within a quarter-mile of the blaze.

In a press conference later, Lt. Karl Pflughaupt, public information officer for the Cy-Fair volunteer fire department, said firefighters "backfilled" the break in the pipeline with a form of nitrogen, he said.

"The fire burned out on its own," Pflughaupt said, though eight trucks had to work the grass fires until after 9 p.m., Pflughaupt said.

Residents were advised to shelter-in-place for about an hour and a half.

"The reason for it was that we wanted residents to stay in, just in case there were any airborne contaminants," Pflughaupt said.

Some people voluntarily evacuated, but that number is unknown, he said.

"As of now, there are no known contaminants and and it is safe for the firefighters to work on the brush fires," Pflughaupt said.

There was heavy traffic and "complete gridlock" near U.S. 290, Martin said.

Residents in the area reported hearing a loud explosion, "a rumbling sound" and seeing a bright orange "fireball."

"The house started to move. It was like an earthquake. Then we opened the door and saw an orange-yellow light and lots of smoke going up," said Carlos Gener, a resident of the Enchanted Rose subdivision.

Kathy Johnson said she was driving to her home in the area when a neighbor called to tell about hearing "a big boom."

"At the time, I was on 290 and Highway 6, and I could see it from there. It was huge," Johnson said. "It was something else, it was huge."

Barbara Bayless, a resident of the 6800 block of Eagle Ridge said she heard "a loud explosion that shook my house" at about 6:30 p.m.

"It was so loud that all of the car alarms went off," Bayless told the Chronicle while the fire was still at its peak. "Now you can hear this giant whooshing sound like a jet engine."

Bayless said that the fire appeared to be out at about 7:45 p.m.

"There's no longer that jet engine sound, and I can't see the big orange glow in the sky anymore," Bayless said.